1. Field of the Invention
The present invention relates to an anti-corrosion treatment of aluminium/zinc alloy surfaces.
In particular, although by no means exclusively, the present invention relates to an anticorrosion treatment of steel strip having a coating of an Al/Zn alloy.
2. Description of Related Art
Zinc, aluminium and/or combinations of aluminium and zinc are widely used as surface coatings, particularly but not exclusively for steel for protection against corrosion. In practice, however, the zinc or Al/Zn coatings are susceptible to white corrosion (white rust) or black corrosion (black rust) respectively when exposed to the atmosphere due to reactions with moisture. Such corrosion is detrimental to the surface appearance and generally makes coated steel substrates unacceptable commercially despite the fact that the overall service life of the coated steel may remain the same. Further the formation of corrosion products generally interferes with finishing operations. The ability to resist such corrosion is referred to herein as wet stack performance.
In order to inhibit the formation of corrosion on coated surfaces it is generally accepted that the treatment of a surface with a chromate imparts anticorrosive properties and this type of treatment is generally referred to as chromate passivation. However, chromate is highly toxic to exposed workers and, due to its high toxicity, disposal of chromium residues is difficult. Further, in various markets yellow discolouration of treated coated surfaces caused by the chromate is considered to be an unacceptable product attribute.
In order to overcome the problems associated with chromate passivation, phosphate coatings have been used. However the anticorrosion properties of phosphate have been found to be far inferior to the above-mentioned chromate treatment.
U.S. Pat. No. 4,385,940 assigned to Kobe Steel, Limited discloses an anticorrosive treatment for preventing white rust on galvanized steel which includes the steps of applying to the surface of a galvanized steel sheet an acidic solution containing molybdic acid or a molybdate in a concentration of 10-200 g/l (calculated as molybdenum) and adjusted to a pH of 1 to 6 by addition of an organic or inorganic acid. However, whilst the anticorrosive treatment described in the US patent works well for galvanised steel, it has been found that the corrosion resistance of Al/Zn alloy surfaces treated with the above solution is inferior to the chromate treated substrates under certain conditions and the treated surfaces suffer from an undesirable degree of discolouration. Moreover, molybdate treated surfaces of such material have been observed to change from a pale yellow/blue to a strong green colour when stored for periods of time in excess of 24 hours.
International application PCT/US97/00012 (WO97/27001) in the name of Henkel Corporation discloses an anti-corrosive treatment for aluminium/zinc alloy surfaces which is based on the use of a solution that includes phosphate anions and molybdenum anions and/or compounds in which the molybdenum has an oxidation state less than +6.
It is an object of the present invention to provide an alternative anticorrosion treatment for Al/Zn alloy surfaces.
According to the present invention there is provided an anticorrosion treatment of an aluminium/zinc alloy surface which includes the steps of:
(i) forming on the surface a coating of a solution which contains 5-40 grams of molybdenum per litre of the solution, 2-19% by volume of a phosphoric acid, and a surface etchant; and
(ii) drying the coating to form a dried coating having a loading of molybdenum of at least 10 mg/m2 of the dried coating and of phosphorus of at least 15 mg/m2 of the dried coating.
The amount of 2-19 volume % of the phosphoric acid equates to 9.1 to 86.9 grams phosphorus per litre of the solution.